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ATLANTA - The CDC said nine pregnant women in the U.S. have contracted lab-confirmed Zika virus disease, and the agency is investigating 10 additional cases.

All 19 women live in the U.S., but had recently traveled to Zika-affected areas, where the virus is being transmitted by mosquitoes.

Since October, the World Health Organization said 5,600 babies have been born in Brazil with microcephaly, or abnormally small heads. Many also had brain damage or other complications.

WHO Director-General Dr. Margaret Chan said the link between Zika virus and microcephaly is becoming stronger, but the virus still hasn't been confirmed as the cause of the birth defects.

"Microcephaly can be caused by other causes,” Dr. Chan said. “It is important to realize that. And the evidence coming over from this country, Brazil points in the direction that Zika is a possible cause. And how does WHO operate? We have been saying this: Zika is guilty until proven innocent."

Of the pregnant women travelers affected here in the U.S., the CDC said four of their nine babies or fetuses either miscarried or showed evidence of birth defects.

But CDC investigators said they didn't expect to see such severe abnormalities in such a small group of women.

Two of the nine women are still pregnant. Two have given birth to healthy infants.

One, a woman in Hawaii, gave birth to a baby with severe microcephaly.

Of the remaining four women, two lost their pregnancy, and two chose to terminate theirs.